A293912 Numbers k such that 2*10^k + 51 is prime.

0, 1, 2, 4, 8, 22, 68, 70, 109, 112, 144, 154, 318, 418, 451, 536, 638, 1540, 1590, 2935, 3859, 8770, 9860, 70688, 133955, 161766, 166458

Offset

1,3

Comments

For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 0 followed by the digits 51 is prime (see Example section).

a(28) > 2*10^5.

Links

Table of n, a(n) for n=1..27.

Makoto Kamada, Factorization of near-repdigit-related numbers.

Makoto Kamada, Search for 20w51.

Example

2 is in this sequence because 2*10^2 + 51 = 251 is prime.
Initial terms and associated primes:
a(1) = 0, 53;
a(2) = 1, 71;
a(3) = 2, 251;
a(4) = 4, 20051;
a(5) = 8, 200000051; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[2*10^# + 51] &]

Crossrefs

Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.

Sequence in context: A158324 A002075 A337720 * A122623 A124014 A100827

Adjacent sequences: A293909 A293910 A293911 * A293913 A293914 A293915

Keyword

nonn,more,hard

Author

Robert Price, Oct 19 2017

Extensions

a(25)-a(27) from Robert Price, May 30 2018

Status

approved